BYU played 5 away games, where we can compare the opponents attendance numbers in non-BYU FBS home games, vs their attendance numbers in BYU games

• Biggest differential was GA Southern, with their game against BYU drawing 4.75k more visitors than the rest of their home schedule (consisting of Louisiana, Ark St, Georgia St and Coastal Carolina)
• Next was USU with a differential of +3.98k more visitors than the avearge non-BYU home game for USU.  Given the proximity and mild rivalry, this is less meaningful though.
• Baylor was the median team with the BYU game drawing 3.48k more attendees than their average home game.  Some (but probably not all) of this difference is also attributable to it being Homecoming.
• USC had an artificially high home average due to the UCLA game.  Including UCLA, BYU game drew 367 fewer attendees, but if that is removed, BYU drew 2k more than the non-UCLA average.
• For the Wazzu game, BYU drew 2k fewer attendees than average and was the least attended game of the year. 

Bottom line, it looks like (compared to what Utah got for the USC game) that BYU brings about 2k extra attendees for away games.  If it's the first time in a region in a while (see GA Southern) then it might be higher.

So not a massive contingent, but the trend is pretty clear (except for Wazzu for some reason)